The present invention relates to a control unit for an electrical power translating device which is constituted by a converter which is used for converting AC to DC and an inverter for inverting DC to AC; more specifically, the invention relates to a technical measure which is suitable for suppressing a beat phenomenon associated with rectification ripples due to rectification by a converter when an AC motor is driven at variable speeds by an inverter.
In an electrical power translating device of a converter-inverter having an intermediate DC stage, when an AC power source for the converter is, in particular, a single phase source (for example, as in an electric rolling stock for an electric railway in which power is fed from an AC trolley line), a ripple frequency component having a frequency twice the frequency of the AC power source is contained in the converted DC voltage due to the rectification. Although, such a ripple frequency component can be reduced if the capacity of a smoothing capacitor to be provided at the DC stage is increased, however, it is difficult to remove the ripple component completely; moreover, with the increase of the capacity of the smoothing capacitor, the size thereof increases, which prevents a reduction of the size and weight of the concerned device.
When the resultant DC voltage containing the above ripple component is inverted by the inverter into a variable frequency variable voltage AC and the inverted AC is supplied to a load, such as an AC motor, the difference and sum components of the above ripple frequency and the inverter operating frequency other than the inverter operating frequency component are contained in the inverter output voltage and the motor current. Among these frequency components, when the operating frequency and the ripple frequency come near each other, the above difference component constitutes a low frequency component, since the impedance in the motor is small in relation to low frequencies. With the low frequency component, a large ripple current flows through the motor, and a beat phenomenon is caused in which the torque generated by the motor ripples.
For example, JA-A-64-77492 discloses the principle as to how the beat phenomenon is caused and a method of suppressing the same. In the beat phenomenon suppressing method disclosed in this patent document, the degree of ripple in the DC input voltage of an inverter is detected, the degree of ripple in frequency for the ripple degree in the DC input voltage is determined with a compensation gain and a compensation phase difference corresponding to an operating frequency, and the inverter frequency is regulated depending on the ripple degree in frequency, whereby the beat phenomenon is suppressed.
However, in the beat phenomenon suppressing method disclosed in the above-mentioned patent document, in order to achieve a high beat phenomenon suppressing effect, it is necessary to regulate the compensation gain and the compensation phase difference depending on the inverter operating frequency. Further, in order to suppress the beat phenomenon always to an optimum condition, it is necessary to take into account a variation of the motor output other than the inverter operating frequency. However, if it is required to regulate the compensation gain and the compensation phase difference by taking into account the inverter operating frequency and the motor output, problems of control complexity and troubles in practice arise.
Further, as disclosed, for example, in JP-A-5-83976, a vector control for an inverter has been recently used in a control unit for an induction motor for driving an electric rolling stock, however, no disclosures in the patent document are found for suppressing the above beat phenomenon by taking advantage of the vector control. Further, the present inventors are aware no other documents relating to suppression of the beat phenomenon.
An object of the present invention is to provide an electric power translating device and a control method therefor which can suppress the beat phenomenon caused by the ripple component contained in the inverter DC input voltage, even if the operating frequency of the inverter and the motor output vary, without necessitating complex gain and phase difference regulations.
The present invention is achieved in an electric power translating device which comprises a converter for converting an AC source voltage into a DC voltage by rectifying the AC source voltage, a smoothing capacitor connected at the DC side of the converter, an inverter for inverting the DC obtained from the smoothing capacitor into an AC and for supplying the AC to an AC motor and means for controlling the voltage, frequency or phase in the AC output from the inverter, by provision of means for regulating at least one of voltage, frequency and phase in the AC output of the inverter based on a ripple frequency component contained in the AC output of the inverter associated with the rectification by the converter.
Further, the present invention is achieved in a control method of an inverter which inverts a DC voltage, formed by rectifying an AC voltage using a converter, into an AC and then supplies the AC to an AC motor by controlling the voltage, frequency or phase in the AC output of the inverter to perform a variable speed control for the AC motor, by regulating at least one of voltage, frequency and phase in the AC output of the inverter based on a ripple frequency component contained in the AC output of the inverter associated with the rectification by the converter in the direction of suppressing a beat phenomenon caused by a rectification ripple.